Competing Interactions of As Adsorption and Fe(III) Polymerization during Ferric Coprecipitation Treatment

This study revealed the effect of As on the formation and dissolution of iron (hydr)oxides and its further impact on the As removal efficacy of FeCl₃ treatment. Adding 6.7 mg/L FeCl₃ into 325 μg/L As solution (coprecipitation) resulted in more As removal (99% As(V) and 75% As(III)) at 2 min than adding As into aged FeCl₃ solution (preaged, 52-87% As(V) and 7-42% As(III)) at pH 7. However, soluble As gradually increased in the coprecipitation system and decreased in the preaged system to give similar concentrations during 800 h aging. The particle size of the iron (hydr)oxides increased more slowly in the coprecipitation than in the preaged systems. These results suggest the rapid adsorption of As on Fe polymer during the initial polymerization process, which delays the growth of iron (hydr)oxides. Thermodynamically, quantum chemical calculations implied that iron ions adsorption on iron (hydr)oxide polymer was more stable than As adsorption, which is the main driving force for the As release during aging process. This study improved our understanding of the kinetic and thermodynamic processes of As adsorption and iron (hydr)oxide precipitation in the coprecipitation treatment of As, and the potential for As release during aging of sludge generated in the treatment.